1. Field of the Invention
The invention relates generally to a vibratory feed mechanism, and in particular, to a feed mechanism which includes an improved amplitude sensing and damping method and apparatus.
2. Prior Art
Systems including vibratory feeder bowls are known for feeding streams of parts or other particulate material. Such a system typically includes a feeder bowl coupled to a stationary base by leaf springs. Relative movement of the bowl and base causes parts within the bowl to move up an incline spiral path and fall into an accumulating container.
In a typical system parts segregated by a vibratory feeder bowl are either weighed or counted to collect a batch of a desired size. Once the batch is complete, the parts are either moved away from the feeder by a conveyor system or are dumped from a first accumulator to a second receptacle and then removed.
As an example, three vibratory feeder mechanisms might be arranged in parallel. A first mechanism would deposit a desired number of bolts onto a conveyor. The second and third feeder mechanisms would send an identical number of nuts and washers to the conveyor to be added to the bolts provided by the first mechanism. In this way a like number of nuts, bolts and washers will be fed from individual vibratory mechanisms and combined to form a batch each containing the proper number of parts. Typically they are then fed to a packaging station.
As the parts are dispensed from the feeder bowl it is desirable that the amplitude of vibration of the feeder mechanism remain approximately constant. It is known that the amplitude of bowl vibration depends upon the mass of materials within the bowl. As the total mass of the bowl contents decreases, a reduction in driving power is necessary to maintain a given amplitude of vibration for the bowl. As the amount of bowl contents increases, the amplitude of vibration will diminish for a given driving power. Proposals have been made to sense the amplitude of vibration of the driven bowl and compensate for changes in the bowl by varying the power acting upon the bowl. The objective of such proposals is to maintain relatively constant vibratory amplitude while parts are being fed to an accumulator.
Prior amplitude sensing techniques have employed inductive elements mounted to the feeder in close relation to a magnet which vibrates with the feeder bowl. As the bowl vibrates, thereby feeding parts in the bowl to a conveyor or packaging station, relative motion between the magnet and an inductor produces an oscillating electrical signal whose magnitude depends upon the amplitude of vibration. This signal has been used to sense the amplitude and control the driving power to the bowl. At small amplitudes of vibration, however, the signal generated in this manner was too small to provide an adequate control signal.
A small amplitude of oscillation is particularly useful in small batch processing where a large amplitude is inefficient since the feeder is continually being started and stopped. Thus, prior art amplitude sensing techniques has been somewhat inadequate when controlling the feeding of small batches of parts.
Another problem with prior vibratory bowl feeders is that they are characterized by inefficient batch feed through due to problems encountered stopping vibrations when a batch has been completed. Prior systems count the number or weigh the mass of units fed from the bowl and seek to terminate the drive power to the bowl when the proper number or weight of units has been fed. A problem has been that when the power has been removed from the driving circuitry, the bowl continues to oscillate or vibrate for a finite period of time due to its inertia and the restoring action of its coupling leaf springs. As the bowl continues to vibrate, the units within the bowl may continue to be fed from it and accumulate in the container. Thus undamped oscillation after power termination may send more than the requisite number of units into an accumulator or container.
Expressed another way, one problem has been that prior feeders tend to over feed. Various expedients have been used to compensate for the over feed problem but the problem itself has continued.
Some prior art systems have dealt with the over feed problem by including a diverter into which the excess parts were fed after forced bowl vibrations were terminated. The excess parts were accumulated and periodically emptied back into the vibratory feeder bowl. These diverter systems were inefficient since the excess parts must be continually returned to the vibratory apparatus and they exhibited other shortcomings. One such shortcoming was repeated recycling could cause excessive wear with some parts and another shortcoming was the diverter would not neccessarily provide the precise flow cut off desired.
A second technique for dealing with the over feed problem was to slow down the oscillations as the requisite number of parts was neared during the feed process. This slowing down of the vibration as the correct part number was neared resulted in a reduced through put for the system. Instead of operating at maximum efficiency for the full cycle for a given batch, the oscillations were slowed as the proper count was neared. This technique also required control circuitry to monitor the number of parts in the accumulator and compare that number with the final count to be achieved.